Synchronization is one crucial subject for digital signal receiving system, which includes symbol timing, frequency synchronization and sample clock synchronization in OFDM system. For the symbol timing which determines the Fast Fourier Transform (FFT) placement location, the proper selection of this location has a major performance impact on all post FFT algorithms. Therefore, it is desirable to achieve good timing early in the acquisition.
In some current systems, e.g. the Orthogonal Frequency Division Multiplexing (OFDM) system, the received signal generally has frame structure as illustrated in FIG. 1, wherein effective symbols are separated by guard intervals (GI) each of which is a copy of an end portion of the effective symbol. Since there is no pilot can be used before the fast Fourier transform (FFT), the symbol timing will be estimated by the cyclic guard intervals.
Conventionally, the symbol timing determination is to calculate the correlation value of the received signal with the received signal delayed by one symbol, detect the peak of the correlation value, and obtain the timing, which is used as the indication for FFT processing initiation for the received signal.
The conventional estimation algorithm is to search maximum correlation peak, which is corresponding to timing position. The basic principle of these algorithms is shift auto-correlation. Since the cyclic prefix i.e. the guard interval is a copy of an end portion of the effective symbol, their correlation has a big correlation peak value. A conventional method of searching the correlation peak can be represented in the following equation:
                                          n            ɛ                    =                      argmax            ⁡                          (                                                Λ                  r                                ⁡                                  (                                      n                    r                                    )                                            )                                      ⁢                                  ⁢                                            Λ              r                        ⁡                          (                              n                r                            )                                =                                                                ∑                                  n                  =                                      n                    r                                                                                        n                    r                                    +                                      N                    g                                    -                  1                                            ⁢                              (                                                      r                                          l                      ,                      n                                                        ·                                      r                                          l                      ,                                              n                        +                        N                                                                                            )                                                                                    (        1        )            
Where, is rl,n is nth received sample of lth symbol, rl,n+N is its N samples delay, nr is starting position of searching, Ng is guard interval length, nε is estimated symbol timing position, and Λr(nr) is likelihood function of received signal at the time nr.
This method is valid under AWGN, ETSI-Rayleigh/Ricean, ETSI-Ricean and some moderately dispersive channel. But in long path channel such as Single Frequency Network (SFN) channel, especially two-path channel whose first arrived path is not the strongest one and the strongest path is far away from the first path, even its channel delay over a guard interval, the conventional estimation algorithm gives a little effort. It only estimates position of average peak or maximum peak, as the result, symbol timing error of the receiver is very big. If the conventional method is used, the first path will be ignored, and Inter-Symbol Interference (ISI) will be generated. Therefore, it is desirable to search the first path in order to avoid ISI and make channel estimation module “see” all Channel Impulse Response (CIR).
There are many studies focus on how to detect the fastest timing in order to achieve good timing as earlier as possible. The synchronization in strong SFN channel can be achieved by using a double correlation method which will find the middle point of the impulse response of channel. As the result, the first useful path is still discarded. If the distance of two path reaches guard interval or even larger, the error will be larger. Moreover the double correlation method needs more memory resource than the single correlation.
EP patent application EP1005204 discloses a symbol synchronization error reduction method aiming to solve the above mentioned problem by selecting a fastest timing among timings detected in a timing detection step during a predetermined time. With reference to FIGS. 2 and 3, in one embodiment of this invention, a digital signal reception apparatus with a fastest timing detection section is provided. FIG. 2 shows a part of the configuration of the digital signal reception apparatus, wherein the received signal is converted into the digital signal in A/D converter 101, and then delayed by one symbol in delayer 102, a multiplier 103 calculates the correlation value of the received signal with the one-symbol-delayed received signal, an integrator 104 integrates the calculated correlation value, a subtracter 105 and a decider 106 executes a decision to detect the peak of the correlation value for each frame of the received signal. With respect to the peaks of the correlation values output from the decider 106, a fastest timing detection section 107 compares the peaks for each of a plurality of received frames and output the fastest symbol synchronization timing as the FFT processing initiating timing. FIG. 3 shows a part of conventional configuration of the fastest timing detection section 107, wherein a counter 201 suspends counting operation at the peak detected timing for each frame of the received signal, a switch 202 switches the counted numerical value for each frame to output to respective memory 203, and a comparator calculates the stored numerical value and output the least numerical value as the fastest symbol synchronization timing.
As described above, this invention aims to provide a digital signal reception apparatus which improves the accuracy for the symbol synchronization acquisition and reduces the error possibility under multipath environment. However, it's still methodologically complicated, so that the memory resource needs for the computation is high.
Therefore, it is desirable to develop an improved method for symbol synchronization of the digital signal reception, which overcomes drawbacks of the prior arts.